The grand telescope's fifth and final repair expedition will involve five spacewalks and extended exposure in a debris-filled region. NASA will take extra precautions to protect the shuttle crew.

After 19 years of service, during which time it has provided the most eye-popping images ever of galaxies, nebulae and, most recently, of a planet orbiting an alien star, the Hubble Space Telescope is suffering the pains of old age. It's unsteady, with only half its gyroscopes working, and several of its key science instruments are broken.

To restore the ailing telescope to its former glory, NASA on Monday is set to launch the fifth and final repair mission to the orbiting telescope.

The Hubble mission is unusually risky, even by space travel standards, involving five spacewalks and extended time in the debris-riddled layer above Earth, where even a small collision with space junk could render the shuttle useless. The vulnerability of the shuttle fleet to small bits of flotsam was tragically demonstrated by the destruction of the shuttle Columbia in 2003, which was hit during liftoff by a piece of foam insulation from the external fuel tank. The searing heat of reentry widened the hole on the left wing and destroyed the orbiter as it attempted to land in Florida.

Top NASA officials say they will do everything they can to ensure the safety of the crew of the shuttle Atlantis, including flying the orbiter upside down and backward to minimize the danger of being struck by space debris and having a second shuttle on the launchpad in case a rescue mission must be mounted.

"This is going to be an extremely challenging mission," said shuttle pilot Greg Johnson.

Big Bang quest

If successful, the mission will leave the telescope with six new gyroscopes, six fully charged batteries, and four repaired or replaced cameras and spectrographs, including the workhorse wide-field camera No. 2 that was responsible for some of Hubble's most dramatic images. The repairs will keep Hubble functioning at peak efficiency at least through 2014, by which time the next-generation James Webb telescope is scheduled to take its place.

This suite of upgraded instruments will enable the refurbished telescope to look back in time to the very beginnings of the universe.

"This will be our first realistic chance of detecting the first stars and galaxies that formed" right after the Big Bang, almost 14 billion years ago, said UCLA astronomer Matthew Malkan, whose team will be among the first users of the refurbished instrument.

Malkan estimates that the reborn Hubble will be 100 times as powerful as it was when it was carried to orbit in April 1990.

It might even be powerful enough to find the current Holy Grail of cosmology -- the source of the dark matter and dark energy that is thought to make up as much as 96% of the universe. Although neither has been detected directly, scientists believe they must exist to explain the behavior of galaxy clusters and the fact that the expansion of the universe is speeding up.

"I truly believe that 100 years from now people will still remember Hubble and what it did," said David Leckrone, a senior scientist with the Hubble team.

The mission is not without controversy. In fact, it was canceled after the Columbia disaster, out of concern that a problem with Atlantis could leave the crew stranded in space. Then-NASA Administrator Sean O'Keefe declared at the time that the only safe destination for the shuttle fleet was the International Space Station, which could offer a haven to the crew of a damaged shuttle.

It took the intervention of Congress, which earmarked money to NASA with the restriction that it could be used only on a Hubble repair mission, and the appointment of a new administrator, Michael Griffin, to get the mission back on the manifest.

"I thought this was a mission we ought to be doing," said Griffin, who recently left the agency for a teaching position at the University of Alabama in Huntsville. "For the cost of one shuttle mission, we get a chance to have a brand new telescope in Earth orbit."

With most of its instruments hobbled or out of service, Hubble today "is like a champion athlete playing hurt," Leckrone said.

On the to-do list

NASA's schedule calls for Atlantis to rendezvous with Hubble on the third day of the 12-day mission. Mike Massimino is one of several crew members who has visited Hubble before. As a member of the 2002 mission, he recalls the thrill of catching a glimpse of the big telescope floating languidly in space.

"As you come up, it looks like a star on the horizon," Massimino said.

After pulling alongside Hubble, crew member Megan McArthur will operate Atlantis' onboard grappling arm to haul the school-bus-sized, 44-foot-long, 12-ton telescope into the shuttle's payload bay, where it will be attached to a special berthing mount that will allow easy access to all its instruments.

Day 4 is when the real work begins, with the first of the five spacewalks, each lasting more than six hours.

The repair team will have at its disposal a kit of 180 tools, 116 developed just for this mission. And 316 bolts will be loosened and refastened as the spacewalkers wrestle the new instruments into position.